1. Field of the Invention
The present invention relates to an optical communication system, and more particularly, to a wavelength division multiplexing-passive optical network (WDM-PON).
2. Description of the Related Art
Fiber To The Home (FTTH) technology is being actively studied and developed all over the world for connecting a home to a telephone office using an optical fiber transmission line so as to provide integrated services including voice calls, data services, and broadcasts. The use of FTTH technology will dramatically increase in the next several years.
It is important, in terms of FTTH technology, to develop optical signal transmission methods for constructing cost effective optical networks at a high productivity. Optical networks can be classified into passive optical networks (PONs) and active optical networks (AONs). Recent PON systems include asynchronous transfer mode-passive optical network (ATM-PON), broadband-passive optical network (B-PON), gigabit-passive optical network (G-PON), and Ethernet-passive optical network (E-PON) systems. In recent AONs, local area networks including Ethernet switches are connected using optical transmission lines.
In wavelength division multiplexing (WDM) based FTTH networks (i.e., WDM-PONs), each subscriber is assigned a particular wavelength and communicates with a central office (CO) or an optical line terminal (OLT) using the assigned wavelength. Therefore, independent, high data-rate, high-security services can be provided to subscribers. Furthermore, unlike time division multiple access-passive optical networks (TDMA-PONs), modulation and demodulation of optical signals are individually performed for each subscriber, so that optic sources having a low modulation speed and power can be used together with optical receivers having a narrow bandwidth.
However, in a WDM-PON, a pair of optical transmitter/receiver modules is required for each subscriber. That is, it takes significant costs to construct a WDM-PON as compared with existing optical networks since optical transmitter/receiver modules should be installed in a subscriber premises and a central office. Moreover, since optic sources having particular wavelengths are required in proportion to the number of subscribers, it is expensive to construct a WDM-PON. In addition, spare optic sources having different wavelengths should be stored for respective subscribers so as to replace broken optic sources. This can be a great difficulty for a service provider in terms of management of wavelengths. Therefore, there is a need for inexpensive WDM-PON optic sources. Furthermore, it is necessary to provide a wavelength-independent optic source for subscribers so as to realize a cost-effective WDM-PON.
Meanwhile, management of wavelengths of optical network terminals (ONTs) is traditionally considered to be important. However, in a WDM-PON that recent service providers want, wavelengths of downstream optic sources of a central office should be easily managed as well as wavelengths of optical network terminals (ONTs) that are easily managed. That is, in a conventional WDM-PON based on a reflective semiconductor optical amplifier (RSOA), identical ONTs are used regardless of wavelengths assigned for the ONTs so as to solve wavelength management problems of the ONTs. However, more recent service providers want a WDM-PON designed to eliminate wavelength management problems of a central office as well as ONTs so that a network can be economically and stably managed.
Specifically, in a WDM-PON, a central office communicates with subscribers using different wavelengths for different subscribers. Therefore, the number of subscribers to the WDM-PON is limited since available wavelengths are limited. Although the number of subscribers to the WDM-PON can be increased by increasing the number of ports of an optical multiplexer (MUX) and demultiplexer (DMUX) and the number of single-mode optic sources (requiring wavelength control) of the central office, this method is uneconomical and also has a limitation in terms of increasing the number of subscribers. That is, wavelengths of a central office as well as ONTs should be efficiently used for more efficient optical communications.
Meanwhile, although data can be transmitted at 1 Gbps or at a higher rate using each wavelength, the high data transmission rate is not fully used since appropriate contents are not yet developed. That is, there is a need for a method of making more efficient use of wavelengths.